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The present invention relates generally to electronic moisture sensors, and specifically to time domain reflectometry moisture sensors. This invention represents modifications and extensions to the method and apparatus for extrapolating soil moisture and conductivity disclosed in U.S. patent application Ser. No. 09/945,528.
A variety of sensors have been developed to detect moisture in various media. These include conductivity sensors, bulk dielectric constant sensors, time domain reflectometer or transmissometer (TDR or TDT) type sensors, and various oscillator devices, the majority of which exploit the high dielectric constant of water to extrapolate moisture content in the medium. In particular, TDR type sensors have been used over the past several years to measure the water content in various applications. Such applications include detecting volumetric soil moisture, determining liquid levels in tanks, and determining moisture content in paper mills and granaries.
A major setback in determining volumetric moisture content in a medium is the influence of conductive materials in the medium of interest. For example, soil conductivity is a function of the ion content of the soil and of its temperature. Salts from irrigation water and/or fertilizer can build up in the soil and cause significant errors in TDR-based moisture readings.
Because of the uncertainty in moisture readings caused by conductivity, many of the TDR sensors now available are xe2x80x9crelativexe2x80x9d sensors. This means that the sensor does not report absolute moisture content readings, but uses reference points obtained through testing. In essence, the moisture sensor does not report absolute moisture content readings, but reports a xe2x80x9cwetter thanxe2x80x9d or xe2x80x9cdrier thanxe2x80x9d condition based on the relative difference of the conductivity-dependent moisture content reading and the reference reading.
Unfortunately, the readings from these xe2x80x9crelativexe2x80x9d sensors do not remain in synchronism with the true or xe2x80x9cabsolutexe2x80x9d water content of the medium, but fluctuate with time. For example, the salinity (ionic content) of soil may fluctuate with season. In such a case, the original reference point becomes an inaccurate indicator of the moisture level of the medium.
The method and apparatus disclosed in U.S. patent application Ser. No. 09/945,528 (""528) provide a way to report absolute volumetric water content of a medium. This is done by essentially analyzing the distortion effects on a transmitted waveform caused by the properties (namely conductivity and dielectric constant) of the medium. The ""528 disclosure provides a means to launch a fast rising positive edge onto a transmission line passing through a specific length of soil. The previously disclosed embodiment and associated method may be modified to suit other configurations and implementations so as to more readily adapt the technique to other media in addition to soil. In a first set of alternatives, since the described system includes both a transmitter and receiver, some variations may be made in how the transmitter and receiver are physically related to one another within the moisture sensing system. A second set of alternative configurations, independent of the first, derives from variations in the manner in which the transmission line is terminated.
The embodiment described in ""528 uses a transmission line that folds back to a receiver mounted on the same circuit board as the transmitter. As a result of housing the transmitting and receiving electronics on the same circuit board, and folding the transmission line, feed-through noise is inherent in the characteristic received waveform. One possible variation from the previously described embodiment is to incorporate what may be referred to as a Bi-static approach.
With the Bi-static approach the transmitting and receiving circuitry are housed on separate circuit boards, connected by a straight unshielded transmission line used for sending the successive waveforms, a shielded transmission line used for timing, and a wire bundle for communication and power purposes. This eliminates the feed-through noise in the characteristic received waveform, resulting in a simpler detection scheme for bulk propagation delay and distorted rising edge slope.
Another alternative embodiment uses a reflected wave rather than the transmitted one. When using the reflected wave approach the transmitter launches a step function at one end of a transmission line, the other end of which may be either shorted or open-ended. The fast rising step function propagates along the line and is reflected at the shorted or open end back to the point of origin. A receiver samples and digitizes the returning waveform into closely spaced digital samples representing the amplitude at precise time intervals of the returned waveform. Analysis of these samples yields an accurate measurement of the round-trip propagation time of the step function, even in the presence of waveform distortion caused by conductive elements in the medium surrounding the transmission line. From the propagation time the bulk dielectric constant of the medium can be determined and from that the volumetric moisture content of the medium. Further analysis of the distortion of the waveform yields the bulk electrical conductivity of the medium.
The disclosed invention is a method and apparatus for inferring volumetric moisture content and bulk conductivity of a medium of interest using a TDR-based system after the manner of the disclosure in ""528. The present invention describes alternative embodiments which use a Bi-static approach in one instance, and reflected wave approaches in other instances, to measure the propagation time.
In all embodiments as in ""528, a very precise timing and successive approximation amplitude-measuring scheme captures the timing of the received waveform with picosecond resolution and its amplitude with millivolt resolution. From point-by-point measurements, the characteristic received waveform is examined. Propagation delay of the characteristic received waveform is set as the first time when the amplitude of the received waveform is greater than a threshold. This information is used to infer the bulk dielectric constant of the moisture-bearing medium. The maximum slope of the characteristic received waveform is also examined and used to infer conductivity of the medium under test.